For example, when manufacturing a silicon wafer, a single-crystal silicon ingot is sliced by using a wire saw apparatus (see, e.g., Japanese Patent Application Laid-open No. 85053-1996). The wire saw apparatus is an apparatus which enables a wire (a high-tension steel wire) to travel at a high speed and presses an ingot against the wire to be sliced (cut) while applying a slurry having GC (silicon carbide) abrasive grains dispersed in a liquid to the wire. As an apparatus for slicing an ingot, one which applies a tension of 2.5 to 3.0 kgf to a wire having a diameter of 0.14 mm to 0.18 mm and performs slicing while enabling the wire to travel in a reciprocating direction at an average speed of 400 to 500 m/min in a cycle of 1 to 2 c/min (30 to 60 s/c) is general in a conventional technology.
The wire is unreeled from one wire reel and enters groove rollers through a tension giving mechanism constituted of one or both of a dancer roller including a dead weight or a powder clutch having a constant torque motor connected thereto. The groove roller is a roller having a structure where a polyurethane resin is pressed in a peripheral part of a cast steel cylinder and grooves are formed on a surface thereof at a fixed pitch, and the wire is wound around such rollers for 300 to 400 turns and then taken up by the other wire reel through the other tension giving mechanism. The wire is driven in the reciprocating direction in a predetermined cycle by a groove roller driving motor. On the other hand, the ingot is pressed against the wire at a preprogrammed ingot feed speed and sliced.
FIG. 2 is a schematic view for explaining a mechanism for feeding an ingot in a conventional wire saw apparatus. In this wire saw apparatus 10′, an ingot 2′ is held to a work feed table 3′ guided by an LM guide (a linear-motion guide) 4′ and sliced by using a wire row 1′ wound around groove rollers 5′. The LM guide 4′ is generally constituted of a “track rail” and a “slide unit including a steel ball or a cylindrical runner”, and the slide unit linearly and smoothly moves on the track rail when the steel ball or the cylindrical runner which is in contact with the track rail rotates.
On the other hand, a size of a surface waviness component called a “nanotopography” is a problem in wafers in recent years. This nanotopography is obtained by taking out a wavelength component of λ=0.2 to 20 mm whose wavelength is shorter than “sori” or “warp” but longer than “surface roughness” from a surface shape of a wafer, and it is very shallow waviness having a PV value of 0.1 to 0.2 μm or below. It is said that this nanotopography affects a yield of an STI (Shallow Trench Isolation) step in device manufacture.
Although the nanotopography is formed at processing steps (slicing to polishing) of a wafer, it has been revealed that the nanotopography due to slicing using a wire saw (i.e., slice waviness) can be classified into three types, i.e., “one which suddenly occurs,” “one which occurs at start or end of slicing,” and “one having periodicity”. Of these three types of nanotopography, it can be considered that the “one which extemporaneously occurs” is caused due to wire disconnection or an error in tension control of the wire saw apparatus, whilst the “one which occurs at start or end of slicing” is caused due to a sudden change in a cut-resistance, often resulting in rejection in nanotopography numerical value judgment. On the other hand, the “waviness having periodicity” often results in rejection in nanotopography sensory test, but its cause is unknown.